The invention relates to apparatus for providing optoelectronic transversal filters, in particular an optoelectronic transversal filter with variable gain bipolar taps.
Transversal filters operate by passing the signal to be filtered through multiple paths such that the propagation times of the signal through each path differs, usually by a constant time interval. The delayed versions of the signal are then recombined to form the output signal. The magnitude or proportion of each delayed signal is controlled in the combination process to provide control over the filter properties. Transversal filters may be of the "finite impulse response" type or the "infinite impulse response type", depending on whether or not the output signal is re-injected into the delay paths.
Transversal filters require delay paths which retain adequate bandwidth to pass the signal to be filtered, and methods of launching and recovering signals onto these paths. A widely used technology is that of surface acoustic waves (SAW). SAW transversal filters employ a single delay propagation path from which signals are tapped at intervals along its length. The input signal to the filter is piezoelectrically launched as an acoustic wave in a suitable crystal. The weighted recombination process of the transversal filter consists of the recovery of the acoustic signal using multiple piezoelectric electrodes distributed along the path of the acoustic wave. Because the propagation velocity of the acoustic wave is relatively slow, significant delays can be obtained in a physically small device.
SAW filters cannot be used for very wideband signals for several reasons. First, the bandwidth of a transversal filter is set by the time interval between taps, which becomes shorter for wider bandwidth filters. For very wideband filters, the slow propagation of the acoustic wave becomes a disadvantage because the taps become too close together for practical implementation. Second, the propagation of the acoustic wave may be sufficiently dispersive that insufficient bandwidth is available in the delay path itself. Current limitations on advanced SAW filters indicate a center frequency of 1 GHz with a 30% bandwidth.
To overcome the dispersion and propagation delay constraints of SAW transversal filters, optical fiber delay line transversal filters have been proposed as, for example, the Fiber Optic Delay Line Filter of Hunt et al. disclosed in U.S. Pat. No. 4,128,759 issued on Dec. 5, 1978. In the Hunt apparatus the signal to be filtered is converted into an optical signal by laser or light emitting diode. The optical signal is then injected into a plurality of fiber optic paths that all terminate on a photodector which serves to sum the optical signals and convert them back into an electrical signal. Each optical path of the Hunt apparatus is provided with an independently operable optical attenuator to control the intensity of the light allowed to pass along its associated fiber optic delay line. A variant apparatus is disclosed by Judeinstein in U.S. Pat. No. 4,166,212 titled Recirculating Optical Delay Line issued on Aug. 28, 1979 where a single fiber optic path is used reiteratively to provide multiple delay paths each path having an integer multiplied delay of the basic delay caused by a single passage of the light through the fiber optic delay path.
The foregoing patents typify the fiber optic delay line transversal filters found in the art. The method of summing the delay signals is simply to deliver the light from all delay lines to the same photodector. This approach is problematic in certain instances where high bandwidth is required. The efficient collection of light from multiple sources into a single detector is difficult for high bandwidths because of size constraints in the detector to maintain low capacitance. Moreover, the tap weights of each delay line must be set to predetermined values to obtain the filter effects desired. This is achieved in the prior art by including a device of controllable transmittance, such as a spatial light modulator, in the optical path. Such optical path devices have several constraints associated with them including: limits on the dynamic range of the weights, limits on the resolution of the weights, constraints on the speed with which the weights may be changed or updated. Also, where electrically controllable optical directional couplers are used for controlling the tap weights, the type of fiber optic cable is then restricted to a singlemode fiber. Single mode propagation can result in optical interference if the delays required require fiber optical path differences that are shorter than the coherence length of the optical carrier in the fiber optic path. Where such delays are required, the delay-bandwidth requirements of the filter sought to be implemented could be met using multi-mode fibers which cannot operate with optical directional couplers.